This invention relates to surgical apparatus and methods for performing lamellar keratotomies. More particularly, the invention relates to a positioning ring for temporarily immobilizing the eyeball, or ocular globe, such that the globe""s cornea protrudes therethrough.
In a normal eye, parallel rays of light entering the eye become focused on the retina to create a sharp visual image. Anomalies in the overall shape of the eye, however, may result in image distortion by causing parallel rays of light entering the eye to become focused at a location other than the retina. Hyperopia, or farsightedness, occurs when the front-to-back measurement of the eyeball is too short, causing parallel rays of light entering the eye to focus behind the retina. In contrast, myopia, or nearsightedness, occurs when the front-to-back measurement of the eyeball is too long, causing parallel rays of light entering the eye to focus in front of the retina. Astigmatism occurs when the parallel rays of light entering the eye do not focus at a single point in the eye, but rather have a variable focus due to an aspherical cornea refracting light in a different meridian at different distances.
Glasses or contact lenses usually correct hyperopia, myopia and astigmatism but surgical corrective methods have become quite popular due to the inconvenience and discomfort of wearing glasses or contact lenses. One of these surgical corrective methods is laser-assisted in-situ keratomileusis (LASIK). During the LASIK procedure, a microkeratome is used to perform an incomplete lamellar keratotomy, which leaves a peripheral residue of corneal tissue uncut to act as a hinge. The hinge permits the corneal disk to be lifted for exposing and resecting the stromal layer with a laser. The tissue removed by the laser reshapes the stromal layer so that the stromal layer will sufficiently refract the light rays entering the eye to cause them to focus on the retina, producing a sharp visual image without the aid of glasses or contact lenses. After the laser completes the reshaping of the stromal layer, the corneal disk is folded back into its original position, using the hinge as a guide. Within minutes, the corneal disk adheres itself to the rest of the cornea and the LASIK procedure is complete.
There are many different designs for microkeratomes but certain aspects of their operation are similar. A suction ring is first affixed to the sclera and centered about the cornea so that the cornea extends through an aperture and above the suction ring. The diameter of the aperture in the suction ring through which the cornea extends is selected on the basis of the size of the cornea and the diameter of the corneal disk to be cut. The suction ring is held to the sclera by a vacuum induced in the area between the cylindrical ring and the eyeball. A float head having a flat, arcuate or oblique surface is then moved over the guide ring so as to compress the cornea into a shape that complements the surface. A cutting head carrying a blade is then moved across the suction ring so as to resect a corneal disk. The float head may be held stationary once it has compressed the cornea prior to the movement of the blade, or the float head may be moved with the blade head while maintaining contact with the cornea. The cutting blade is moved a predetermined distance substantially, but not completely, across the portion of the cutting plane that intersects the cornea. The movement of the cutting blade is restricted by an adjustable stop means, whereby the extent of hinge width formed on the corneal disk is adjustable. The cutting blade is then moved back across the suction ring to its original position, so that the suction ring can be removed and the resulting corneal disk can be folded back and secured over its hinge, exposing the corneal stroma to be reshaped by the laser.
While the use of the hinge has made corrective surgery easier and safer by helping to ensure that the corneal disk is replaced in its original position, the hinge has the disadvantage that it can potentially block a section of the ablation zone covered by the corneal disk hinge. FIG. 1 shows a plan view of an ocular globe (eyeball) with the corneal disk 11 folded back over the hinge 14, exposing the stromal layer 12 and the laser ablation zone 13 after an incomplete lamellar keratotomy. For a corneal disk cut with the existing microkeratomes, only about 60% to 70% of the circle diameter of the corneal disk may be safely used because of the interference caused by the hinge. In the example shown in FIG. 1, a 10 mm diameter corneal disk 11 is cut and folded back over the hinge 14. The resulting laser ablation zone 13 has a diameter of only 6 mm because the hinge area plus a safety zone required to protect against laser damage to the hinge, is 2 mm wide and, because both the corneal disk 11 and the laser ablation zone 13 are circles, the same 2 mm of the diameter in the counter-lateral quadrant, or the opposing quadrant furthest from the center of the eye, cannot be used for a circular laser ablation. Therefore, in this example, of the original 10 mm diameter, only 6 mm can be used for laser ablation.
Using a smaller suction ring aperture diameter to reduce the diameter of the corneal disk may be especially harmful if the stromal zone exposed by the microkeratome is smaller than the laser ablation zone, as then ablation may occur over the hinge. This type of ablation damage, referred to as negative hinge syndrome, creates or produces an irregularity of the ablation with an inevitable astigmatic induction, loss of visual acuity, and a decrease in contrast sensibility.
To prevent negative hinge syndrome, surgeons performing LASIK will either obtain unnecessarily large corneal disks or move the suction ring toward the position where the hinge will be located to gain more stromal tissue exposure for the ablation. Obtaining unnecessarily large corneal disks increases the risk of complications from the higher vacuum required under the suction ring to hold the larger suction ring in place on the sclera during the incomplete lamellar keratotomy procedure. Using the higher vacuum increases the intraocular pressure, thereby placing eye structures, such as the retina, at risk. Moving the suction ring toward the position where the hinge will be located to gain more stromal tissue exposure creates a change in the anatomical coupling of the eye with the ring, therefore increasing the risks of suction loss and serious corneal damage during the incomplete lamellar keratotomy procedure.
The suction ring must be securely affixed to the sclera and the corneal region by the suction induced by a vacuum pump. On some eyes, the eyeball is aspherical to a larger degree than normal making it difficult to achieve a good suction seal between the sclera and the suction ring. A cornea with a higher degree of astigmatism will also make it more difficult to seal the corneal region against the aperture on the top end of the suction ring. The suction ring is a cylinder with a circular aperture on the top end through which the cornea is exposed for cutting. The bottom end of the suction ring is also circular and attaches to the sclera by suction induced by a vacuum, normally pulled through ports around the inside wall of the suction ring. A small vacuum pump is used to create the vacuum. To acquire the necessary suction to hold the suction ring in place on the eye, a seal must be maintained both around the bottom end of the suction ring with the sclera as well as the top aperture of the suction ring with the corneal region. This is illustrated in FIGS. 2, 3 and 4.
FIG. 2 shows a properly fitted suction ring on an eye. A good suction seal is formed between the corneal region and the top aperture 25 and the sclera and the bottom end of the suction ring 24. The suction chamber 21 is the volume inside the cylinder formed by the suction ring and sealed by the eye. The vacuum is induced through port 22 to the vacuum pump.
FIGS. 3 A-B show a poorly fitting suction ring at the top aperture. In this example, the cornea is misshapen due to astigmatism such that the suction ring, in FIG. 3A, does not adequately seal between the corneal region and the top aperture 25 along the a-axe2x80x2 meridian. However, in FIG. 3B, the suction ring is well sealed between the corneal region and the top aperture 25 along the b-bxe2x80x2 meridian. FIGS. 4 A-B show a poorly fitting suction ring at the bottom end of the suction ring. In this example, the eyeball is aspherical causing, as shown in FIG. 4A, a poor suction seal between the sclera and the bottom end of the suction ring 24 along the a-axe2x80x2 meridian. However, in FIG. 4B, the suction ring is well sealed between the sclera and the bottom end of the suction ring 24 along the b-bxe2x80x2 meridian.
The majority of refractive surgery cases have an astigmatic cornea of less than 2 diopters between one meridian and the other. When the cornea is extended into the top aperture of the suction ring, the flatter meridian tends to curve and the more curved meridian tends to flatten to conform to the suction ring. The suction produced by the vacuum pump helps the cornea to adjust and fit into the aperture. However, when the astigmatic cornea is greater than 2 diopters between one meridian and the other, or the difference in radii between one meridian and the other is greater than about 1 mm, the risk of losing the suction seal when the cornea is flattened and pushed downwards with the float head increases and, if greater vacuum is used to seal the suction chamber, the structures of the eye are at risk as the intraocular pressure increases. Thus, as the cornea""s astigmatism increases, the risk of having suction problems during the procedure also increases.
Therefore there is a need for a suction ring shaped in such a way as to ensure a good suction seal both around the top aperture for an astigmatic cornea as well as a good suction seal around the sclera for an aspherical eyeball. It would be very advantageous to have a microkeratome that greatly reduced the risk of suction loss between the eyeball and the suction ring while performing an incomplete lamellar keratotomy procedure. It would be desirable to have a device for performing an incomplete lamellar keratotomy that would produce a hinge that does not occlude a large area from the laser ablation area.
The present invention provides a suction ring for immobilizing an ocular globe during a surgical procedure on the cornea. The suction ring provides an inferior engaging surface and a superior engaging surface that engages and grips the ocular globe, immobilizing the ocular globe relative to the surgical procedure. The inferior engaging surface grips or engages the sclera of the ocular globe while the superior engaging surface grips or engages the corneal region. The engaged corneal region is the cornea and one portion of the limbo, the zone that joins the cornea and the conjunctiva. The suction ring also provides an aperture sized to receive and expose the cornea for a surgical procedure.
The inferior and superior engaging surfaces are secured against the ocular globe by suction induced by a slight vacuum pulled by a vacuum pump or other vacuum source. Between the inferior and superior engaging surfaces is an annular vacuum channel that is connected to the vacuum pump via a tube. When suction is applied by pulling a slight vacuum, the ocular globe is slightly pulled into the channel between the inferior and superior engaging surfaces, resulting in the eye being xe2x80x9cpinchedxe2x80x9d and securely held in place relative to the suction ring for the surgical procedure.
The present invention is a suction ring used for a lamellar keratotomy during LASIK refractive correction. The suction ring has a shape more easily adapted to aspherical ocular globes and astigmatic corneas. The suction ring immobilizes the ocular globe in relation to the microkeratome by gripping the sclera and corneal region through suction imposed by a vacuum pump. It is vitally important that the suction grip be maintained during the lamellar keratotomy to prevent the blade on the microkeratome from accidentally cutting into unintended structures of the eye and severely damaging them. When a cornea has an abnormal amount of astigmatism or an ocular globe is abnormally aspherical or ellipsoidal, the risk of suction loss during the lamellar keratotomy increases because the circular suction rings currently used do not have a sufficiently close fit to these shapes to form a reliable suction seal needed to grip the sclera and corneal region. The present invention uses a suction ring having a gripping structure that provides a closer fit to abnormally shaped ocular globes and corneas.
The present invention also provides non-circular apertures for receiving and exposing the cornea for the surgical procedure. The non-circular aperture enables the surgeon to obtain a non-circular corneal disk during the lamellar keratotomy procedure with a microkeratome using a horizontal cutting movement across the cornea.
The present invention also provides a microkeratome, used for performing a lamellar keratotomy, with the suction ring of the present invention.